1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to a perpendicular magnetic recording disk having a permeability gradient. More particularly, the perpendicular magnetic recording disk is formed such that a soft magnetic underlayer (SUL) structure in the disk is formed in a way that the permeability of the SUL structure increases from an inner radius of the disk to an outer radius of the disk.
2. Statement of the Problem
One type of recording medium presently used in magnetic recording/reproducing apparatuses is a longitudinal magnetic recording medium. A longitudinal magnetic recording medium includes a magnetic recording layer having an easy axis of magnetization parallel to the substrate. The easy axis of magnetization is the crystalline axis that is aligned along the lowest energy direction for the magnetic moment. Another type of recording medium is a perpendicular magnetic recording medium. A perpendicular magnetic recording medium includes a magnetic recording layer having an easy axis of magnetization oriented substantially perpendicular to the substrate.
The perpendicular magnetic recording medium is generally formed with a substrate, a soft magnetic underlayer (SUL), an interlayer, a perpendicular magnetic recording layer, and a protective layer for protecting the surface of the perpendicular magnetic recording layer. The soft magnetic underlayer (SUL) serves to concentrate a magnetic flux emitted from a main pole of a write head and to serve as a flux return path back to a return pole of the write head during recording on the magnetic recording layer. The interlayer serves to control the size of magnetic crystal grains and the orientation of the magnetic crystal grains in the magnetic recording layer. The interlayer also serves to magnetically de-couple the SUL and the magnetic recording layer.
Perpendicular magnetic recording media is typically in the form of a circular disk having concentric circular tracks for storing data. Disk drive manufacturers strive to achieve higher and higher areal densities on the disk. To achieve the higher areal densities, track densities and the linear densities of the disks are increased. As track density increases, it becomes important to control the track width precisely. A precisely-controlled track width depends on the magnetic write width (MWW) that can be achieved on the disk, which is the width of bits in the radial direction.
It is a problem to maintain a uniform MWW at different radii of the disk. The MWW of a perpendicular magnetic recording disk is a function of the data recording frequency. The data recording frequencies for writing to a perpendicular magnetic recording disk vary depending on the circumference of the track in an attempt to have a uniform linear density throughout the disk. For instance, the tracks closer to the outer radius of the disk are written with higher recording frequencies (i.e., higher data rates) than the tracks closer to the center of the disk so that the tracks have a substantially similar linear density. However, the MWW of the bits becomes smaller at higher recording frequencies, which makes the MWW larger for the inner tracks (i.e., lower data rates) as compared to the outer tracks (i.e., higher data rates). It would be desirable to fabricate perpendicular magnetic recording disks that allow for a substantially uniform MWW throughout the disk.